1. Field of the Invention
The present invention relates to a data code transmission device, and more particularly to a data code transmission device with a function of correcting the phase of data codes to be transmitted.
2. Description of the Related Art
Various types of digital interfaces have been developed and used for data transfer between a personal computer and its peripheral devices. They include parallel interfaces such as the Small Computer System Interface (SCSI) and serial interfaces such as the Universal Serial Bus (USB). While those standard interfaces are not particularly fast, they offer sufficient performance in transferring text files and other kinds of lightweight data objects.
The recent advancement of multimedia technologies including image data compression techniques has enabled personal computer users to manipulate digital video data on their machines. Many people today take video snapshots with their own camcorders. Such personal video users may wish, for example, to view their videos on a computer screen while uploading the data from a digital camcorder. However, the existing interfaces such as SCSI and USB mentioned above are not suitable for such realtime, large data transfer applications. The multimedia needs in recent years have thus led to the emergence of a new generation of high-speed serial lines or buses. The high-speed serial lines transport data serially at a high bitrate over a single transmission line.
Typical applications of recent high-performance serial lines include connections on a backplane (a circuit board having connectors and slots to serve as a backbone to connect several cards together to make up a complete electronic system). A new serial backplane connection offers a data transfer speed of as high as 3 Gbps, and further development is under way for faster interface.
Besides serving as an interface for personal computer peripherals, high-speed serial transmission techniques facilitate internal transport of signals in various systems. One application is a radar device that detects and determines the distance of objects by using a spread spectrum technique. This device emits a radio wave modulated with spreading codes, analyzes a reflected signal with a correlation detection algorithm, and determines the object distance from the power of correlation-detected values. High-speed serial lines are used here to distribute internally generated serial spreading codes to correlation detection circuits inside the device.
Typical devices for high-speed serial interface include serializers for parallel-to-serial conversion and deserializers for serial-to-parallel conversion, which are collectively referred to as SerDes devices. Serializers consolidates a plurality of low-speed parallel signals into a single high-speed serial signal, whereas deserializers recover clock and data signals from a received high-speed serial signal to reconstruct the original parallel data signals.
Data bits transmitted over a plurality of serial lines, however, may not necessarily reach the destination at the same time; the signals may have different phases at the receiving end. Since those phase differences degrade transmission quality, an appropriate correction technique has to be implemented in such multi-channel systems so that the serial lines will be in phase with each other or locked in a specified phase. Several researchers have proposed the use of buffers to temporarily store a plurality of signals in the form of cells, the read timings for which are controlled such that their outputs will have no phase differences. See, for example, the Japanese Patent Application Publication No. 6-164623 (1994), paragraphs [0009] to [0027] and FIG. 1.
While the phase of each signal channel may be adjusted at the transmitting end, but the characteristics of transmission lines (e.g., backplane) would affect the phase of individual signals before they arrive at the destination. For this reason, most conventional systems are designed to correct phase error on the receiver side. SerDes devices are not an exception. Their phase difference compensation function is implemented only in deserializers used at the receiving end. The lack of phase correction capability at the sending end is a problem for such systems that use received high-speed serial signals without serial-to-parallel conversion and thus have no deserializer devices. This leads to a demand for development of serial data transmitters with phase correction functions.
The aforementioned Japanese Patent Application Publication No. 6-164623 discloses bitwise phase correction based on the difference detected in synchronization with clock pulses. The bitwise method, however, does not work for phase differences smaller than one bit time, such as those derived from transmission line characteristics. Also, the conventional clock-based design for phase difference detection circuits is unsuitable for serial interface with speeds of several gigabits per second, since it requires a clock frequency much higher than the transmission rate.
FIGS. 15A and 15B illustrate a problem of a phase difference smaller than a single bit time. Specifically, FIG. 15A shows data bits with a delay of two bit times with respect to reference data, which can be corrected by a process of bit-by-bit phase difference correction. FIG. 15B, on the other hand, shows a delay of one and half bits. Conventional phase difference correction, however, is unable to correct error of a fraction of bit time.